Head and Neck squamous cell carcinomas (HNSCCs) are highly aggressive multi-factorial diseases affecting 600,000 patients worldwide each year. Alcohol, tobacco, and HPV exposure are known causative factors for HNSCCs, but HPV- HNSCCs represent about 80% of the incidence and are associated with poor clinical outcomes, especially those that manifest a mesenchymal phenotype. Despite the major advances of PD-1/PD- L1 and CTLA-4 checkpoint inhibitors in providing curative benefits for some cancer patients, the response rate in HNSCC patients to these checkpoint inhibitors is limited. Besides the high levels of heterogeneity and immune cell plasticity, a highly immunosuppressive tumor microenvironment (TME) is another contributing factor to the poor clinical outcomes of HNSCCs, which underscores the need for defining additional immune checkpoints non- redundant to the PD-1/PD-L1 pathways. CD73 is a major ecto-5'-nucleotidase and a rate limiting enzyme that converts the immune stimulatory ATP to adenosine (ADO), which inhibits T cell activation and induces apoptosis. Thus, CD73 serves as a prevalent immune checkpoint for maintaining immunosuppression. Clinically, high CD73 expression in total HNSCC tissues has been implicated in poor prognoses. Nevertheless the major contributor of CD73 activity in the HNSCC tumor microenvironment (TME) has not been identified and should be addressed by rigorous and comparative examination of each cell type because the observed elevated CD73 expression in total tumor tissues could be a result of either an increase in CD73 levels in one cellular subtype or an increase in the frequency of this cell type. Recently, we observed that cancer-associated fibroblasts (CAFs), the prominent non-hematopoietic stroma in the TME, expressed the highest levels of CD73, whereas CD73 expression was highly variable in other cellular constituents of the HNSCC TME. Furthermore, immune cell plasticity in HNSCCs, including the loss of T cells in the TME, was associated with high CAF abundancy and high CAF-CD73 levels. Given the highly heterogeneous cellular constituents of HNSCCs, we propose that a comprehensive comparative analysis of CD73 expression in different cell types in the context of their relative abundancy and bio-distribution will provide more insight into the major contributor(s) of the total CD73 activity and the effects of CAF abundancy to immune cell plasticity. Using fresh HNSCC specimens and state-of-the-art technical approaches, we will address the crucial questions of whether and how CAF-CD73 modulates immune cell plasticity with two specific aims: (1) to define and correlate the CD73 expression levels on CAFs with immune cell plasticity via FACs and multiplexed IHC; and (2) to demonstrate the plasticity of CAFs and immune cells in the HNSCC TME via transcriptome profiling of purified tumors, CAFs, and TILs from typical HNSCCs and mesenchymal-like HNSCCs, as well as to demonstrate their differential CD73 enzymatic activity and immunosuppressive function. The results of our systemic and comprehensive analyses will allow us to construct an overall immune landscape of HNSCC TME and to provide invaluable information about the dominant contributing population(s) to the CD73 checkpoint.